It has been recognized that in thin solar cells, a reduction in the surface recombination velocity of the cell back surface will produce a reduction in the diffusion current generated in the back layer of the cell, thereby improving the cell open circuit voltage as well as increasing the collection of light generated carriers from this back region to thereby increase the short circuit current. One such technique for reducing the surface recombination velocity of a germanium solar cell is taught by Beck in U.S. Pat. No. 3,620,829, where the active surface of the germanium device is coated with antimony trioxide.
It has also been shown theoretically and verified experimentally that a p.sup.+ back layer as illustrated in FIG. 1 will reduce minority carrier transport between the p layer and the back surface and will accordingly improve the solar cell short circuit current and open-circuit voltage. The p.sup.+ layer 4 is deposited on the p layer 2 of an n.sup.+ p type cell to form a back layer and is more heavily doped than the p layer of the cell. For an n.sup.+ p cell, a p.sup.+ back layer is deposited on the p layer and will be used as an example throughout the specification. However, the same analysis is equally true for an n.sup.+ layer on a p.sup.+ n cell, a p.sup.+ layer on an n.sup.+ i cell, or an n.sup.+ layer on a p.sup.+ i cell. The letter "i" indicates intrinsic material or high resistivity material that acts intrinsic under normal operating conditions. Deposited on the p.sup.+ back layer is an ohmic contact 6 provided to collect current generated by light. The structure consisting of the p.sup.+ back layer 4 and the ohmic contact 6 will herein be designated a "p.sup.+ back contact."
Previous attempts to improve p.sup.+ back contacts, such as the JPL Contract No. 954,833 to Solarex, have been only moderately successful. In those attempts an aluminum or aluminum bearing layer has been diffused into the whole back surface of solar cell. The result is expected to be a p.sup.+ layer formed from the diffused aluminum and terminating on the back side in an ohmic contact at the silicon-aluminum alloy layer.
A different type back layer is taught in U.S. Pat. No. 3,888,698 "Infrared-Transparent Solar Cell" in which an open metallic contact grid deposited on a p.sup.+ back layer is utilized to allow non-useful long wavelength light to pass through the back surface rather than be absorbed in the back layer. An oxide coating is deposited on the remaining exposed portions of the p.sup.+ layer to improve the thermal emissivity of the cell.
Theoretical analyses have been conducted on p.sup.+ back contacts. See M. P. Godlewski, C. R. Baraona, H. W. Brandhorst, "Low-High Junction Theory Applied to Solar Cells," Conference Record, Tenth IEEE Photovoltaic Specialists Conference, Palo Alto, CA, November 13-15, 1973, p. 40 and P. M. Dunbar and J. R. Hauser, "A Theoretical Analysis of the Current-Voltage Characteristics of Solar Cells," Annual Report on NASA Grant NGR-34-002-195, August 1975, and NASA CR-134740, February 1975.
Optimization of the front structure for a shallow junction cell to reduce the front layer surface recombination velocity on using a minimum contact area is taught in U.S. Pat. No. 3,982,964 "Dotted Contact Solar Cell." The ohmic contacts are deposited directly on the n+ front layer in a manner to provide minimum contact between the electrode and the semiconductor material.